The cell mediated immunity, particularly a cytotoxic T cell (hereinafter, referred to as “CTL”) plays a significant role in the in vivo rejection of cancer cells or virus-infected cells. CTLs recognize a complex between an antigen peptide (“cancer antigen peptide”) derived from a cancer antigen protein and an MHC (major histocompatibility complex) class I antigen, which is referred to as “HLA antigen” in the case of human, on a cancer cell, and attack and kill the cell.
A cancer antigen peptide which binds to MHC class I molecule is generally known to be a peptide having 8-12 amino acid residues produced by intracellular processing of the protein. Thus, in general, a peptide having 8-12 amino acid residues derived from a cancer antigen protein can be a candidate for a cancer antigen peptide. When a glutamine residue or a cysteine residue is present in the cancer antigen peptide, those amino acid residues are spontaneously oxidized in air atmosphere in general. It is reported that such spontaneous oxidization decreases binding affinity of the peptide for MHC class I molecule and cognitive response to the peptide by T cell receptor (see nonpatent literatures 1 and 2).
The tumor suppressor gene WT1 of Wilms tumor (WT1 gene) has been isolated from chromosome 11p13 as one of the causative genes of Wilms tumor based on analysis of the WAGR syndrome that occurs as a complication of Wilms tumor, aniridia, urogenital abnormalities, mental retardation and so forth, and the amino acid sequence of WT1 is publicly known (see nonpatent literature 3). The WT1 gene is expressed with high frequency in human leukemia, and when leukemia cells are treated with WT1 antisense oligomers, the growth of the cells is inhibited. Thus, WT1 gene is thought to act to promote the growth of leukemia cells. Moreover, WT1 gene is also highly expressed in solid cancers such as gastric cancer, colon cancer, lung cancer, breast cancer, embryonal cancer, skin cancer, bladder cancer, prostate cancer, uterine cancer, cervical cancer and ovarian cancer, and the WT1 gene has been demonstrated to be a novel cancer antigen protein in leukemia and solid cancers (see nonpatent literatures 4 and 5). In addition, a cancer antigen peptide having a partial sequence of WT1 protein that is a wild type cancer antigen peptide was identified (see patent literatures 1 and 2).
Particularly, WT1235-243 (Cys-Met-Thr-Trp-Asn-Gln-Met-Asn-Leu; SEQ ID NO: 1), that is a peptide spanning in positions 235 to 243 of the cancer antigen protein WT1, is a cancer antigen peptide having an activity to induce CTLs in HLA-A24-restricted manner (see nonpatent literature 6 and patent literature 1). The modified peptide (Cys-Tyr-Thr-Trp-Asn-Gln-Met-Asn-Leu; SEQ ID NO: 2, hereinafter it may be referred to as WT1235-243 (2M→Y)), in which the methionine residue at position 2 of WT1235-243 is replaced with tyrosine residue, has a higher binding affinity for the HLA-A24 antigen than the wild type peptide (see patent literature 3). The development of both wild type peptide WT1235-243 and the modified peptide WT1235-243 (2M→Y) as an immunotherapeutic agent is promising.
In addition, it is known that each of said wild type peptide and said modified peptide has a cysteine residue at the N-terminal site, oxidization of which in air atmosphere produces the dimer bound via disulfide bond, and said dimer may also work as a cancer antigen peptide (see patent literature 4).    Patent literature 1: WO 00/06602    Patent literature 2: WO 00/18795    Patent literature 3: WO 02/079253    Patent literature 4: WO 2004/063217    Nonpatent literature 1: Immunity., 6:273, 1997    Nonpatent literature 2: J. Immunol., 160:2099, 1998    Nonpatent literature 3: Cell, 60:509, 1990    Nonpatent literature 4: J. Immunol., 164:1873-80, 2000    Nonpatent literature 5: J. Clin. Immunol., 20, 195-202, 2000    Nonpatent literature 6: Clin. Cancer. Res. 8: 2626, 2002